PHP is a computer programming language that runs on ‘back end’ web servers. Coding aficionados may turn their nose up at PHP, but it’s popularity cannot be disputed. Sites that were built on PHP include Facebook, Wikipedia and Mailchimp, and the world's most popular CMS - Wordpress - is written in it.
PHP allows you to insert dynamic content on your website. So a really basic example would be that you could use PHP to check the time of day, and you could set the title of your webpage to be “Good morning!” if it’s before midday or “Good afternoon!” otherwise. PHP is also good for connecting to databases. So if you ran a webpage that sold cars, and a user went to a page with the URL ‘https://www.example.com/cars-in-londing.php’ the PHP script on the web server would connect to a database, and ‘query’ it for all cars that are being sold in London. It would then ‘loop’ through these results, building up an HTML page to send back to and show to the end user. This is the traditional way to create websites with dynamic content, and although there are now lots of other ways to achieve the same end it still does the trick.